In many industries, like the petrochemical and chemical industries, contact of reaction fluids with a catalyst in a reactor under suitable temperature and pressure conditions effects a reaction between the components of one or more reactants in the fluids. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
Many arrangements seek to overcome the negative effects of endothermic chilling by supplying heat to the reaction or of exothermic heating by removing heat from the reaction. More traditional methods employ multiple stages of heating between adiabatic reaction stages. Other methods use in-situ heating via simultaneous reactions or indirect heat exchange to maintain an isothermal or other temperature profile within the reaction zone. U.S. Pat. No. 5,525,311 provides an example of indirect heat exchange with a heat exchange fluid to control the temperature profile within a reaction zone.
A variety of processes can employ indirect heat exchange within a reaction zone to control temperature profiles within the reaction zone. Common examples of hydrocarbon conversion reactions include: the aromatization of hydrocarbons, the reforming of hydrocarbons, the dehydrogenation of hydrocarbons, the oxidation of hydrocarbons and the alkylation of hydrocarbons. Most of these hydrocarbon conversion processes process streams having high concentrations of hydrogen.
It is known to accomplish indirect heat exchange for processes with a variety of heat exchanger configurations including shell and tube heat exchange designs or thin plates that define reaction and heat exchange channels. In such arrangements the tubes typically contain catalyst while the channels contain a heat exchange fluid or in a plate arrangement the channels alternately retain catalyst and reactants in one set of channels and a heat transfer fluid in adjacent channels. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. A specific arrangement for heat transfer and reactant channels that offers more complete temperature control can again be found in U.S. Pat. No. 5,525,311; the contents of which are hereby incorporated by reference. Other useful plate arrangements for indirect heat transfer are disclosed in U.S. Pat. No. 5,130,106 and U.S. Pat. No. 5,405,586.
High heat capacity heat transfer fluids are used in several industries to provide cooling for shell and tube heat exchanger arrangements. Various types of high heat capacity fluids include alkali liquid metals such as sodium, lithium, and potassium and include molten salts such as nitrates and carbonates. These heat transfer fluid combine high heat capacity with high thermal conductivity. British patent 2170898 generally discloses the use of sodium as a heat transfer medium in high temperature reactions including heat recovery from furnace installations, high pressure nuclear reactors, coal gasification, coal conversion, and water disassociation. U.S. Pat. No. 4,549,032 discloses the use of molten salt as an indirect heat transfer medium with a dehydration of styrene. German patent DE 2028297 discloses the use of an alkaline metal as a heat transfer medium in a process for producing alkenes and aromatics by cracking aliphatic hydrocarbons. The liquid metals are specifically used due to their high heat transfer capacity that permits utilization of small heating surfaces.
When indirectly heating or cooling hydrocarbons or other chemical feeds, the presence of hydrogen poses special problems for the use of liquid metals and other high heat capacity heat transfer fluids. Any finite hydrogen activity requires some provision for removal of metal hydride that will form from hydrogen that constantly permeates through the walls of the barrier between the fluids. Should the metal hydride concentration exceed solubility limits, the precipitation of solid hydride can interfere with the operation of the process or cause damage to equipment. Where the hydrogen permeation rate is small a chemical sorbent or getter material is used to chemically react and bind the hydrogen to prevent saturation of the metal hydride and its subsequent precipitation into the circulating system. Also the nuclear industry has used cold traps for many years to removal small quantities of sodium hydride.
Many hydrocarbon and petrochemical processes have a much higher hydrogen partial pressure on the process side of the heat exchange surfaces than the usual processes in which liquid sodium and other heat transfer fluids have been used. In many hydrocarbon conversion processes, the problem of hydrogen permeation can be severe. Many such processes work best with a relatively high hydrogen partial pressure which directly influences the problem of hydrogen permeation. Furthermore, obtaining a highly efficient heat exchange benefits from an increase in the surface area for the indirect heat exchange. As a surface area increases relative to the flowing fluid volume, the permeation of hydrogen into the liquid metal also increases. The recent trend in heat exchange arrangements for hydrocarbon conversion processes is to use a series of thin stacked plates which maximizes surface area, but at the same time, greatly increases the hydrogen permeation rate, particularly for those processes that maintain a relatively high hydrogen to hydrocarbon ratio. Therefore, it is particularly desirable to have a process that can simply and effectively control the concentration of hydrides in the liquid metal heat exchange fluid. Typical cold traps that remove hydride precipitate or getters would quickly reach their capacity limit with the high hydrogen permeation rates associated with the chemical process. Replacement of cold traps and getter material will be prohibitively costly and inconvenient.
Those skilled in the art of using liquid metals as indirect heat exchange materials have addressed the problem of eliminating impurities, in particular, hydrides from the liquid metal streams. U.S. Pat. No. 4,713,214 shows a de-gassing chamber for purifying liquid metal coolant from a fast neutron nuclear reactor using a filter element that provides the primary means of purification and a de-gassing chamber that collects bubbles of an inert gas blanket that may become entrained in the circulating liquid sodium. U.S. Pat. No. 4,581,200 uses a tank in combination with a cold trap wherein the tank deposits a sodium mist in contact with hydrogen to act as a hydrogen getter for subsequent intermediate release of hydrogen by heating of the sodium deposit. U.S. Pat. No. 4,290,822 discloses a method for cleaning a cold trap that uses sodium hydroxide to transform heated impurities into liquid phase and then draining off the liquid phase that may use vacuum conditions to remove any possible traces of water. U.S. Pat. No. 3,941,586 also teaches the purification of cold trap by heating sodium hydride to a molten state and removing or venting hydrogen gas from the cold trap. The typical apparatus associated with a cold trap comprises an economizer exchanger that transfers heat between hot, unpurified metal and the cold purified metal, a cooler for the liquid metal, and some form of retainer for a filtering element, or metallic fibers. It is known from U.S. Pat. No. 4,713,214 that cold trap devices may be integrated in a reaction vessel or may be external to the reaction vessel and involve a secondary circulation loop. U.S. Pat. No. 4,290,822 discloses the heating of cold traps with resistance heaters to maintain a temperature of about 355.degree. C. to dissolve sodium hydride and sodium hydroxide. None of these methods are particularly suited for process fluids that have a high hydrogen concentration.
It is, therefore, an object of this invention to provide a method of removing hydride from circulating liquid metal heat exchange fluids that can accommodate a high hydrogen permeation from the process fluid.
It is a further object of this invention to provide a simplified system of removing hydrogen and the resulting metal hydride from a circulating liquid metal heat transfer fluid that facilitates the regeneration of traps for further purification of the circulating liquid metal material.